The present invention relates to a linear predictive analysis-by-synthesis (LPAS) encoding method and encoder.
The dominant coder model in cellular application is the Code Excited Linear Prediction (CELP) technology. This waveform matching procedure is known to work well, at least for bit rates of say 8 kb/s or more. However, when lowering the bit rate, the coding efficiency decreases as the number of bits available for each parameter decreases and the quantization accuracy suffers.
[1] and [2] suggest methods of collectively vector quantizing gain parameter related information over several subframes. However, these methods do not consider the internal states of the encoder and decoder. The result will be that the decoded signal at the decoder will differ from the optimal synthesized signal at the encoder.
An object of the present invention is a linear predictive analysis-by-synthesis (LPAS) CELP based encoding method and encoder that is efficient at low bitrates, typically at bitrates below 8 kbits/s, and which synchronizes its internal states with those of the decoder.
This object is solved in accordance with the appended claims.
Briefly, the present invention increases the coding efficiency by vector quantizing optimal gain parameters of several subframes. Thereafter the internal encoder states are updated using the vector quantized gains. This reduces the number of bits required to encode a frame while maintaining the synchronization between internal states of the encoder and decoder.